Follow-up to #466: staging and preview services attach only to the
externally-named trails-shared network (created by the production
compose project) — they have no per-project default network, so the
enable_ipv6 added to the staging file was dead config. Move the flag
to trails-shared where it takes effect.
Rollout requires recreating trails-shared on the flagship: detach
staging, previews, and production postgres, remove the network, re-up.
Production journal/planner are unaffected (they use the default
network).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>